Micromachined electrolyte sheet, fuel cell devices utilizing such, and micromachining method for making fuel cell devices

ABSTRACT

A sintered electrolyte sheet comprising: a body of no more than 45 μm thick and laser machined features with at least one edge surface having at least 10% ablation. A method of micromachining the electrolyte sheet includes the steps of: (i) supporting a sintered electrolyte sheet; (ii) micromachining said sheet with a laser, wherein said laser has a wavelength of less than 2 μm, fluence of less than 200 Joules/cm 2 , repetition rate (RR) of between 30 Hz and 1 MHz, and cutting speed of preferably over 30 mm/sec.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates in general to ceramic electrolytes and fuel celldevices utilizing them, and to the laser micromachining of electrolytesheets and electrolyte supported multi-cell solid oxide fuel celldevices.

2. Technical Background

The present invention pertains to articles formed by laser processing ofsolid oxide fuel cell electrolyte sheets, as well as manufacture ofelectrolyte supported solid oxide fuel cells and fuel cell devices.

Solid oxide fuel cell devices incorporating flexible ceramic electrolytesheets are known. In such fuel cell devices, often one or moreelectrolyte sheets are supported within a housing, on a frame, orbetween a pair of mounting assemblies, which might be either a frame ora manifold. The electrolyte sheets may be utilized either in amulti-cell or single cell design.

A common approach utilizes a fuel cell device that consists of a singlecell design where the thickest component of the fuel cell is an anodelayer. This anode layer acts as both support and catalyst and can beabout 100 to 1000 microns in thickness and is often formed from acomposite of nickel and yttria stabilized zirconia. Such single cellsfurther include a thin electrolyte layer overlying the anode layer, anda cathode layer overlying the electrolyte.

In a multi-cell design, such as that disclosed in U.S. Pat. No.6,623,881 assigned to Corning Incorporated, the fuel cell deviceincludes an electrolyte sheet in the form of a thin ceramic sheet (e.g.,zirconia doped with yttrium oxide (Y₂O₃)). The zirconia basedelectrolyte sheet may be 20-30 microns thick. Typically, the dopedzirconia electrolyte sheet supports a plurality of cells, each of whichis formed by an anode and cathode layer on either side of the dopedzirconia sheet. The thin pre-sintered electrolyte sheet can supporteither a single anode and cathode pair, thereby forming a one celldevice, or multiple anodes and cathodes and a plurality of cells arefabricated on a common electrolyte substrate and are interconnected,through the thickness of the electrolyte sheet by the conductive viaconnectors (vias).

In order to avoid fracturing of electrolyte sheets, the fuel cell devicefabrication process typically utilizes mechanical punching of the viaholes and mechanical cutting of the device edges while the electrolytesheet is in the un-fired state. The process of mechanically punching ofunfired ceramic electrolyte sheets requires predicting the sinteringshrinkage of a particular electrolyte batch in particular furnaceconditions. If the prediction is off, the punched via holes will bemisaligned after sintering. After punching and cutting, the electrolyteis fired and typically undergoes 15% to 30% linear shrinkage due to thede-binding and sintering process. Larger electrolyte pieces requirebetter accuracy in shrinkage values to maintain the tolerances neededfor device fabrication, especially with multi-cell devices. For example,an electrolyte length of 50 cm and via hole positioning tolerances of+/−200 μm in the sintered state, corresponds to predicting theelectrolyte shrinkage by better than +/−0.05%. Mechanically punching andcutting of the un-fired electrolyte puts limitations on the fabricationspeed, feature size, wrinkle, and edge quality produced. Also, machiningof parts in the un-fired state requires an accurate prediction of partshrinkage in order to maintain dimensional tolerances. Such predictionis very difficult to do with the desired accuracy and require actualdevices to be sacrificed for testing.

The general use of laser micromachining thick ceramics is known. It isapplicable to machining of bulk ceramic pieces with thickness of 250 μmor larger, and not thin electrolyte films of thickness of less than 50μm. Thin (less than 50 μm) zirconia based sintered electrolyte sheetsare brittle when they are either cut or/and drilled by mechanical means,due to crack formation.

The process of forming via holes in sintered ceramic substrates forelectronic components is described in U.S. Pat. No. 6,270,601. Thispatent discloses use of either mechanical or laser drilling of thicksintered ceramic substrate with thicknesses of 3-60 mils (76.2 to 1524μm). This reference suggests that laser drilling of sintered ceramicpieces may be achieved by using either CO₂ or excimer laser systems. Nodetails are provided on how to laser machine via holes in sinteredelectrolyte sheets. Applicants attempted to utilize CO₂ laser indrilling thin zirconia ceramic electrolyte sheets, but were notsuccessful due to a large number of cracks created by thermal effects.U.S. Pat. No. 6,270,601 also provided no guidance on how to utilizeexcimer laser for successful cutting or drilling of the electrolytesheets.

US patent publication No 2002/0012825 describes a fuel cell electrolytesheet with 3-dimensional features micromachined on its surface. Thisapplication does not teach or suggest that it is possible to lasermachine electrolyte sheets after sintering.

Prior efforts to produce flat electrolyte of thicknesses greater than 50μm has led to waves or dimples and edge burrs as described in EuropeanPatent EP 1063212B1. This reference discloses stacking of electrolytesheets during sintering, to limit the wave and burr heights to under 100μm. The reference teaches that zirconia sheets and other ceramics sheetsare brittle when subjected to external forces in a bending direction. Incontrast, fuel cells formed from thin flexible electrolyte can withstandsignificant bending without failure. However, they also develop edgecurl when sintered, and the edge curl can produce stress, and fracturethe sheet when the curl is flattened.

SUMMARY OF THE INVENTION

This invention utilizes laser micromachining of the sintered electrolytesheets and fuel cell devices in order to cut the electrolyte sheetsand/or fuel cell device components to size, to trim the edges ofsintered electrolyte sheet edges or fuel cell devices, and/or to producevia holes and surface modifications or patterns.

According to one aspect of the present invention a sintered electrolytesheet comprises: a body of no more than 45 μm thick, and laser machinedfeatures having at least one edge surface exhibiting at least 10%ablation. According to one embodiment, this edge surface exhibits morethan 50% fracture and less than 50% ablation.

According to one embodiment of the present invention a method ofmicromachining an electrolyte sheet comprises: (i) supporting a sinteredelectrolyte sheet; (ii) micromachining the electrolyte sheet with alaser wherein said laser has a wavelength of less than 2 μm, fluence ofless than 200 Joules/cm², and repetition rate (RR) between 30 Hz and 200KHz. Preferably, the cutting speed greater than 30 mm/sec. Preferably,the laser wavelength is less than 400 nm, repetition rate (RR) between30 KHz and 200 KHz. In some embodiments, laser fluence is less than 30Joules/cm². According to some embodiments, the laser micromachingprocess combines the ablation and auto-cleaving (auto-fracturing) into asingle occurrence and increases the cutting speed capability. Accordingto some embodiments, the laser is a n laser (pulse duration <1 μs, forexample 1 to 100 ns). According to some embodiments, this laser is a 355nm laser.

The disclosed method is applicable to manufacture of features formed bylaser micromachining electrolyte supported multi-cell fuel cell devices,and is applicable for manufacturing of flexible electrolyte supportedSOFC devices. As stated above, it is applicable to cutting, shaping anddrilling of solid oxide fuel cell electrolytes of less than 45 μm thickand enables novel designs and manufacturing processes of the fuel celldevices.

One advantage of the present invention is that it advantageously allowsfabrication of new fuel cell designs, and/or advantageously increasesthe fabrication yield and strength of the current fuel cell devices.More specifically, the speed, placement accuracy, and resulting qualityof the electrolyte sheet edge(s) enable flexibility in device design,handling, and improved electrolyte sheet edge strength. Preferably, thesurface roughness of the laser micromachined regions is less than 0.5 μmRMS, more preferably less than 0.4 μm RMS. Preferably this surfaceexhibits peak-to-valley roughness of less than 5.5 μm, or Ra surfaceroughness of less than 0.3 μm. Fuel Cell devices can also be drilled,cut, or micromachined at various times during the fabrication processresulting in fuel cell devices with unique attributes such as complexperimeter shapes or via hole patterns, electrodes or other layersexisting up to the electrolyte edge, and thin electrolyte areas lessthan 5 μm thick. This micromachining process can be utilized at anydesired time after the electrolyte sintering, enabling flexibility indevice fabrication. The resulting method advantageously results indevices and electrolyte sheets that have surprising improvements toflatness and strength.

According to one embodiment of the present invention, laser machining ofsingle and multi-cell devices may be performed after sealing or mountingof the fuel cell device(s) in a support or manifold structure, andresults in improved: electrolyte sheet edge strength, device flatness,edge quality, fewer and smaller wrinkles, minimized electrolyte sheetedge curl, and process yield and throughput. According to anotherembodiment, the micromachining may also be performed to cut electrolytesheets, where multiple fuel cell devices are patterned (e.g., printed)on a single electrolyte sheet and the electrolyte sheets are thenoptionally laser machined to produce multiple devices, thereby savingtime and labor.

The laser micromachining method advantageously enables micromachining ofthe electrolyte in the sintered state, instead of before firing. Thiseliminates the need to accurately predict shrinkage during binderburn-out and sintering. It also eliminates the need for this shrinkageto be uniform across the entire electrolyte sheet.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A illustrates the absorption of zirconia as a function ofwavelength.

FIG. 1B illustrate schematically the process of scoring an electrolytesheet with a laser beam.

FIG. 1C illustrates a via hole according to one embodiment of thepresent invention.

FIGS. 2 a and 2 b are schematic cross-sectional views of exemplary viaholes achieved with laser micromachining. FIG. 2 a illustrates a viahole drilled through an electrolyte sheet. FIG. 2 b illustrates anexemplary via hole drilled through the electrolyte sheet and an anode.

FIG. 3 illustrates schematically utilization of laser micromachining forcutting out multiple fuel cell devices fabricated on a commonelectrolyte sheet substrate.

FIG. 4 illustrates schematically a cross-sectional view of a fuel celldevice with the anode(s) and cathode(s) situated less than 5 mm fromelectrolyte sheet edge.

FIG. 5 illustrates one laser cutting system according to an embodimentof the present invention.

FIGS. 6 a and 6 b are photographs of an exemplary laser micromachinedvia hole in a sintered electrolyte sheet. FIG. 6 a is a photograph ofthe top surface of the sintered electrolyte sheet (i.e., the surfacewhere laser beam was incident on) and FIG. 6 b is a photograph of thebottom surface of the sintered electrolyte sheet (i.e., the surfacewhere laser beam was exiting from).

FIGS. 7 a and 7 b are SEM photos of laser micromachined edges onsintered electrolyte sheet. FIG. 7 a is a top view (surface where laserwas incident) and FIG. 7 b is a cross-section view of the micromachinededge face.

FIG. 8 illustrates another laser cutting system.

FIGS. 9 a-9 c are SEM images of a laser cut edge of an exemplarysintered electrolyte sheet. FIG. 9 a illustrates a cross sectional viewof a laser cut edge. FIG. 9 b illustrates the edge profile of a lasercut edge (receding away from the picture). FIG. 9 c is similar to thatof FIG. 9 a, but shows higher magnification of a laser cut edgecross-section. It also illustrates effects of individual laser pulses inthe upper ablation region.

FIGS. 10 a-10 c are plots of edge surface roughness of exemplaryelectrolyte sheet edges micromachined by a ns laser.

FIGS. 11 a-11 c are plots of edge surface roughness of exemplaryelectrolyte sheet edges micromachined by a fs laser.

FIGS. 12 a and 12 b are XPS line profiles showing change in relativeyttrium and zirconium concentrations. FIG. 12 a is a line scan startingat mechanically cut and sintered edge.

FIG. 12 b is a line scan starting at a laser micromachined sinteredelectrolyte sheet edge.

FIGS. 13 a-13 c are SEM images of a laser cut edge of an exemplarysintered electrolyte sheet. FIG. 13 a illustrates a cross sectional viewof a laser cut edge. FIG. 13 b is similar to that of FIG. 13 a, butshows higher magnification of a laser cut edge cross-section. It alsoillustrates effects of individual laser pulses in the upper ablationregion. FIG. 13 c illustrates the edge profile of a laser cut edge(receding away from the picture).

FIGS. 14 a and 14 b are XPS line profile showing change in relativeyttrium and zirconium concentrations. FIG. 14 a is a line scan startingat a laser micromachined sintered electrolyte sheet edge. FIG. 14 b is aline scan starting at mechanically cut and sintered edge.

FIGS. 15 a-15 c are SEM images of a laser cut edge of an exemplarysintered electrolyte sheet. FIG. 15 a illustrates a cross sectional viewof a laser cut edge. FIG. 15 b is similar to that of FIG. 15 a, butshows higher magnification of a laser cut edge cross-section. FIG. 15 cillustrates the edge profile of a laser cut edge (receding away from thepicture).

FIGS. 16 a-16 c are SEM images of a laser cut edge of an exemplarysintered electrolyte sheet. FIG. 16 a illustrates a cross sectional viewof a laser cut edge. FIG. 16 b is similar to that of FIG. 16 a, butshows higher magnification of a laser cut edge cross-section. It alsoillustrates effects of individual laser pulses in the upper ablationregion. FIG. 16 c illustrates the edge profile of a laser cut edge(receding away from the picture).

FIG. 17 shows probability plots of edge strength showing Weibulldistributions for mechanically cut and laser micromachined electrolytesamples, measured by 2-point bending.

FIGS. 18 a through 18 c are optical microscope images of exemplary viaholes drilled through sintered electrolyte sheets.

FIG. 19 Illustrates laser pulse patterns utilized in a multipassdrilling approach.

FIGS. 20 a and 20 b are photographs of an exemplary laser micromachinedvia hole in a sintered electrolyte sheet. FIG. 20 a is a photograph ofthe top surface of the sintered electrolyte sheet (i.e., the surfacewhere laser beam was incident on) and FIG. 20 b is a photograph of thecross-sectional surface of the sintered electrolyte sheet.

FIGS. 21 a through 21 f are photographs of other exemplary lasermicromachined via holes in a sintered electrolyte sheet. FIGS. 21 a, c,e are photographs of the top surfaces of the sintered electrolyte sheet(i.e., the surface where laser beam was incident on) and FIGS. 21 b, d,f are photographs of the cross-sectional surface of the sinteredelectrolyte sheet.

FIG. 22 shows edge contour plots of (top) electrolyte sheet aftermechanical cutting and sintering, and (bottom) after removing 2 mm fromedge with laser micromachining.

FIGS. 23 a and 23 b are SEM images of a 60 μm hole micromachined with afemtosecond laser in a sintered electrolyte sheet. FIG. 23 a is aphotograph of the top (laser incident) side and FIG. 23 b is aphotograph of the bottom (laser beam exit) side.

FIGS. 24 a and 24 b are SEM images of edges of sintered zirconiaelectrolyte sheets. FIG. 24 a illustrates an electrolyte sheet edgesurface that was mechanically cut (in green state) before sintering.FIG. 24 b illustrates laser cut (micromachined) sintered edge. Length ofscale bar is 10 μm.

FIG. 25 shows probability plots of strength showing Weibulldistributions for mechanically cut and laser micromachined electrolytemeasured by 2-point bending.

FIGS. 26 a-26 c illustrate schematically laser micromachining of theelectrolyte surface. FIG. 26 a illustrates an electrolyte sheet withsingle electrode layer, FIG. 26 b illustrates a laser micromachinedfeature, FIG. 26 c illustrates a fuel cell device with a secondelectrode situated over the micromachined feature (window).

FIGS. 27 a-27 d are photographs of exemplary laser micromachinedsintered electrolyte surfaces.

DETAILED TECHNICAL DESCRIPTION OF THE INVENTION

According to some embodiments a method for laser micromachining of fuelcell electrolyte sheets 100 and fuel cell devices 150 includes the stepsof: (i) supporting a sintered electrolyte sheet or fuel cell device;(ii) micromachining the electrolyte sheet or the fuel cell device with alaser 160 at a cutting speed of more than 20 mm/sec, preferably morethan 30 mm/sec, and more preferably more than 35 mm/sec; wherein saidlaser 160 has a wavelength of less than 2 μm; optical power of more than2 W, and/or laser fluence of less than 30 Joules/cm²; and repetitionrate (RR) between 30 Hz and 1 MHz. Preferably, the laser wavelength isless than 400 nm and even more preferably less than 300 nm, repetitionrate (RR) is between 30 KHz and 200 KHz. In some embodiments laserfluence is less than 400 Joules/cm², for example 350 Joules/cm² or less,or even less than 26 Joules/cm². In some embodiments the laserwavelength is 355 nm, in other embodiments the laser wavelength is in200 nm-300 nm range.

This method can be successfully applied to flexible ceramic electrolytesheets with a thickness of 45 μm or less and advantageously produces nosignificant microcracking at or adjacent to the micromachined surfaces.For example, this method can be utilized with thin sintered ceramicelectrolyte sheets 100 (e.g., zirconia electrolyte sheets) to producevia holes 102, cut edges 103, and surface patterns 105 (e.g., microwindows). According to some embodiments (described below), this methodresults in electrolyte sheet 100 with laser machined features 102, 103,105 having at least one edge surface 104 with at least 10% ablation(region 110). According to at least one embodiment, this edge surfaceexhibits more than 50% fracture (region 112) and less than 50% ablation.According to some embodiments, the surface roughness of the lasermicromachined edge surface 104 is less than 0.5 μm RMS, more preferablyless than 0.4 μm RMS. Preferably the edge surface 104 exhibitspeak-to-valley roughness of less than 5.5 μm, or Ra surface roughness ofless than 0.3 μm. According to some embodiments, the peak-to-valleysurface roughness of the electrolyte sheet in an area on the lasermicromachined edge surface (at the edge) is 0.2 to 5.5 μm, preferably 1to 5 μm, more preferably less than 4 μm. According to some embodimentsthe laser micromachined surface 104 has average crystal grain size ofless than 1 μm. According to some embodiments the fractured surfaceregion 112 has smaller average crystal grain size than that of thecrystals in the transition zone 114 between the fracture region and theablation region.

Laser ablation and/or laser induced breakdown based micromachining useshigh intensity laser pulses provided by a laser 160 to selectivelyremove material. Laser micromachining can be done, for example, withlong pulse UV lasers, and ultrashort pulse lasers such as pico-second(ps) and femtosecond (fs) lasers. For example, we can utilizepico-second lasers such as the Nd:YVO₄ lasers with a mode-locked seederand power amplifiers, or femtosecond lasers such as Ti:Saphhire laserwith a regenerative amplifier system; as well as ultrafast (i.e., withpulse duration <100 ps) fiber lasers. Nano-second (ns) diode-pumpedsolid state (DPSS) lasers such as 3^(rd) (355 nm) and 4^(th) (266 nm)harmonics of Nd:YAG and Nd:YVO₄ lasers are also well suited for use withthe method of the laser micromachining according to the embodiments ofthe present invention.

FIG. 1 a illustrates relative optical absorption of zirconia basedelectrolyte sheets as a function of laser wavelength. Optical absorptionof zirconia is characterized by relatively little absorption in visiblewavelengths and significant scattering due to the multi-crystal grainnature of the material. Laser micromachining with nanosecond UV (<400nm) or deep UV (<300 nm) lasers has the advantage of producing a smallor insignificant heat-affected zone, and smaller feature sizes thanthose carried out using visible and infrared lasers. Photons from UV ordeep UV lasers are absorbed by the targeted material and have enoughenergy to break down material directly. UV and deep UV laser light canbe focused down to smaller diffraction limited sizes and can machinesmaller features than visible and infrared laser light.

Laser micromachining with ultrashort (<100 ps) pulse lasers also canproduce small features with limited (i.e., small) or no heat affectedzone, although the underlying principle of micromachining operation isfundamentally different. Because ultrashort pulse laser micromachiningis based on nonlinear absorption of light in the material, the targetmaterial does not need to absorb laser light directly. Instead, theelectric field in an ultrashort pulse laser is so extreme that initialunbound electrons in the target material are accelerated to create acascade of free electrons through collisions. The cascade of freeelectrons results in break-down of targeted material. Because ultrashortlaser machining is based on nonlinear absorption, it is not limited bydiffraction. Features as small as tens of nanometers have been machinedusing this method.

Typical techniques used in laser via hole drilling/cutting (i.e., lasermicromachining) applications according to some embodiments arepercussion drilling, trepanning, and helical drilling. In percussiondrilling, the laser focal spot is fixed and a train of laser pulses isused to ablate through the material. Trepanning technique is utilizedfor manufacturing large holes, and it is essentially a percussiondrilling process along a circular path. In contrast to trepanning, thehelical drilling reaches the breakthrough only after many turns ofspiral describing the path of the ablation front. Helical drilling(cutting by laser micromachining) is not limited to circular geometry.Holes of any shape can be made by the use of scanner or translationstages.

Cutting, edge trimming, or hole drilling/cutting of electrolyte sheet orfuel cell devices by laser micromachining may be performed by eithercompletely ablating the targeted material (for example, with afemtosecond laser), or by scoring (via ablation) and fracturing thetargeting material (for example, with a 266 or a 355 nm nanosecondlaser). This is shown schematically in FIG. 1B. The fracturing throughthe thickness of the electrolyte sheet occurs as a result of anauto-cleaving (auto-fracturing) due to thermal stress across the depthof the electrolyte material. When laser micromachining (of electrolytesheets or fuel cell devices) utilizes fracture of the targeted materialvia auto cleaving (such that >50% of the thickness is fractured), thisprocess results in laser cutting speeds of over 30 mm/sec. Smalldiameter hole drilling can be performed using ablation, by utilizinghigh speeds (faster than 30 mm/sec) and low pulse energies (for example,below 60 μJ, or below 50 μJ, 40 μJ, 3 μJ, 20 μJ, 17 μJ or 15 μJ) tominimize microcrack formation around the perimeter of the hole. (SeeFIG. 1C) If laser micromachining of zirconia based electrolyte sheets isdone mostly through ablation (e.g., ablating 90% to 100% of the targetedmaterial), according to some of the embodiments the electrolyte sheet100 has grain growth of less than about 2 μm on the cut (ablated)surface. That is, the grain size at or immediately adjacent to theablated surface is larger than the grain size on another area(non-ablated area) of the electrolyte sheet. For example, a typicalgrain size in a non-ablated surface may be 0.2 to 0.5 μm, while theablated surface may exhibit grain larger grain sizes (e.g., 0.9 μm, 1μm, 1.3 μm, 1.5 μm, or 2 μm).

One advantage of the method of the present invention is that thismethod: (i) advantageously allows fabrication of new fuel cell designssuch as complex non-rectangular electrolyte shapes and via holepatterns, complex non-circular via hole shapes, micromachined thinelectrolyte regions less than 5 μm thick; and/or (ii) advantageouslyincreases the fabrication yield and strength of the current fuel celldevices. More specifically, the speed, placement accuracy, and resultingelectrolyte edge quality enable flexibility in device design, handling,and edge strength. The inventive laser micromachining process can beutilized at any desired time after the electrolyte sintering, enablingflexibility in device fabrication. A typical fuel cell device 150includes an electrolyte sheet 100, at least one and preferably aplurality of electrode pairs 152 (cathodes and anodes), electricalconnectors (e.g., conductive vias situated within via holes), bus bars,and other, optional layers. Fuel cell devices 150 can also beadvantageously drilled, cut, or micromachined at various times duringthe fabrication process resulting in fuel cell devices with uniqueattributes such as shapes, flatness, and strength. The resulting methodadvantageously results in fuel cell devices and electrolyte sheets thathave surprising improvements to flatness and strength (for example,peak-valley flatness less than 50 μm and bend strength greater than 2GPa). For example, according to one embodiment of the present invention,laser machining of single and multi-cell solid oxide fuel cell devicesmay be performed after sealing or mounting of the fuel cell device(s) ina support or manifold structure, which results in improved edgestrength, edge quality, handling, and process yield and throughput.According to another embodiment, multiple fuel cell devices are printedon a single electrolyte sheet, and the electrolyte sheets are then cut(laser micromachined) to separate the fuel devices from one another,thus simultaneously producing multiple devices, thereby saving time andlabor, and increasing throughput.

During the laser micromachining process, either the electrolyte sheet100 (or the fuel cell device 150) can be mechanically moved, or thelaser beam provided by the laser 160 can be scanned across theelectrolyte sheet (or the fuel cell device) for faster processing. Also,the output from one laser can be split into multiple micromachininglaser beams to speed up the write time. Some embodiments of the presentinvention exhibit improved electrolyte sheet edge strength (bendstrength) of greater than 1.8 GPa. Some embodiments of the presentinvention exhibit electrolyte sheet edge strength (bend strength) ofgreater than 2 GPa. This strength is measured in a 2-point bend systemwhere the laser micromachined 2 cm×8 cm electrolyte sheet sample is bentupon itself between approaching parallel plates until it breaks into twoapproximately 2 cm by 4 cm sections.

Improved edges: Mechanical cutting and punching requires close attentionto the cutting tool maintenance to avoid tearing of the electrolyteedges. Laser micromachining of sintered ceramic electrolyte sheets 100according to the embodiments of the present invention is capable ofadvantageously producing a cut edge with an edge surface 104 that hasless roughness, chips, tears, or other stress concentrating featuresthan an edge produced by mechanical cutting. This improves the edgestrength of the electrolyte sheet, and can advantageously reduce thenumber of electrolyte pieces damaged during manufacturing due tobreakage at the edges.

As the electrolyte sheet's dimensions increase, it experiences higherstresses during handling and sintering cycles. The laser micromachinedfuel cell devices 150 and electrolyte sheets 100 with higher edgestrength will result in lower amounts of the electrolyte sheet breakageduring fabrication.

Improved manufacturability and quality: Laser cutting device substrates(electrolyte sheets of the desired size) out of an over-sized sinteredelectrolyte sheet enables the over-sized electrolyte sheet to bepositioned using only coarse mechanical alignment. Since laser via holedrilling and edge cutting of a sintered electrolyte sheet can occurduring the same step, precise alignment to a pre-formed edge is notrequired. In this case, the over-sized electrolyte sheet is placed onthe translation stage (e.g., XY stage or XYZ stage) such that anear-perfect area is selected and laser cut out (micromachined) fordevice fabrication. The ability to adjust the position of fabricatedfuel cell devices makes it possible to avoid electrolyte sheet defects101. The precise location of the cut-out electrolyte sheet piece can beadjusted within the over-sized electrolyte sheet to avoid identifieddefects. This improves overall device quality and process throughput.

Laser micromachining can also be used to cut the sintered electrolyte tothe correct shape (rectangular, circular, or other) and size. Typically,the electrolyte sheet is mechanically cut in the un-fired state and thensintered. Thus, the shrinkage that will occur during sintering needs tobe accurately predicted, which is difficult to do. Laser micromachiningof sintered electrolyte sheets does not require accurate positioning.

For example, the fabricated 10-cell devices 150 with overall dimensionsof 12 cm×15 cm may have tight tolerances for example +/−1 mm in order tofit in the mounting frame. Using laser micromachining to cut theelectrolyte sheet 100 to size eliminates the need for accurate shrinkagecontrol during sintering. Also, the laser micromachining can occur atarbitrary times during the fabrication process. For example theelectrolyte sheet 100 can be cut before, after, or between the separateelectrode material printing/firing steps.

If the electrolyte sheet is mechanically cut to size and punched whileattached to the Teflon carrier film for handling purposes, themechanical cutting and punching damages the carrier film and makes itimpractical to recycle. Because laser micromachining occurs when theelectrolyte sheet is in the fired state, the electrolyte carrier film(e.g., Teflon carrier) can now be continuously recycled.

Complex configurations: Laser micromachining allows fabrication of newfuel cell device designs that are not possible or practical throughmechanical cutting. For example, electrolyte sheets 100 can be laser cutinto complex non-rectangular shapes, any desired patterns, and edges canbe cut in very close proximity to previously printed layers. Achievingthis by mechanical cutting of green (i.e., unsintered electrolytesheets) would require very accurate prediction of part shrinkage duringsintering, which is hard or impossible to achieve in manufacturing(commercial scale).

Laser micromachining does not require a rigid back support likemechanical cutting, and various lens systems or an auto-focusing processcan be incorporated in the setup. This allows via hole drilling and edgecutting to occur on electrolyte sheets with arbitrary surface profiles,contours, and corrugations that would have been difficult or impossiblewith mechanical means. More specifically, the laser beam depth of focusand shape can be modified to either cut corrugated structures with largesurface contours or produce shaped edge profiles. Electrolyte sheetcorrugations are useful for improving the strain tolerance of the deviceas described in U.S. Pat. No. 6,582,845 B2, but are difficult orimpossible to accurately cut or mechanically punch in the green, unfiredstate. The laser micromachining process according to the presentinvention enables cutting and via hole formation in corrugated pre-firedelectrolyte with height variations of greater than 100 μm, 250 μm, oreven 1000 μm or more, which may be useful for strain relief.

Improved vias: Laser micromachining enables efficient production of highquality via holes 102 with diameters of less than 75 μm (e.g., 60 μm, 45μm, 40 μm, 30 μm, 25 μm, or 20 μm), punching/cutting through printedelectrodes or other layers, complex non-circular via shapes, and complexpatterns of via holes.

According to some embodiments, laser micromachining process can be usedto manufacture via holes 102 in sintered electrolyte sheet 100, insteadof mechanical punching in the un-fired state. The process ofmechanically punching of unfired ceramics requires predicting thesintering shrinkage of a particular electrolyte batch in particularfurnace conditions. If the prediction is off, the punched via holes willbe misaligned after sintering. Electrolyte shrinkage during sinteringmay be from 15% to 30%. Larger electrolyte sheets require betteraccuracy in shrinkage values to maintain the tolerances needed fordevice fabrication, especially with multi-cell devices. For example, anelectrolyte length of 50 cm and via hole positioning tolerances (i.e.,registration repeatability) of +/−200 μm in the sintered state,corresponds to predicting the electrolyte shrinkage by better than+/−0.05%. Laser micromachining of the via holes 102 in sinteredelectrolyte sheet 100, however, eliminates the need to accuratelypredict electrolyte shrinkage during sintering because, after sintering,no distortion of the electrolyte will occur to misalign the via holepatterns.

Practical mechanical punching of via holes limits the hole diameter toapproximately 75 μm minimum after firing. Although mechanical punches of50 μm are known, lifetime of the mechanical punch at these diameters isvery small. Laser micromachining the via holes 102 allows a practicalreduction in hole diameter (less than 75 μm) as well as the fabricationof arbitrary hole patterns. For example, a pattern of several smallervia holes arranged in clusters can take the place of one 75 μm diametervia hole. The smaller via hole diameters enable more efficient viafilling. Micromachining also enables the creation of via holes 102 atoptimum times in the fabrication process. Holes can even be formed inseveral material layers attached to the electrolyte sheet. For example,holes can be micromachined after the anode layer 103 situated on thezirconia based electrolyte sheet 100 has been printed and fired, whichcreates a continuous via hole through both layers (electrolyte sheet 100and the anode 103) simultaneously. FIGS. 2 a, 2 b respectively,illustrate schematically examples of via holes 102 drilled through theelectrolyte sheet 100 and the electrolyte sheet/anode layers 100, 103.Different via hole cross-section geometries (for example, non-circular)are also possible to achieve by this method.

As a comparison, mechanically punching via holes with diameters of lessthan 75 μm have reduced equipment lifetime due to punch breakage.Smaller than 75 μm diameter via holes with aspect ratio (L/D) of length(electrolyte thickness) to hole diameter of approximately 1:1 (i.e.,0.3:1 to 2:1) will have more efficient filling as well and improveddurability. The smaller via holes 102 filled with the conductivematerial would have reduced voids and defects and therefore haveextended mechanical durability. Larger via holes (larger than 75 μm, andespecially larger than 100 μm in diameter) have a greater tendency toform voids in the via fill on sintering, while smaller via provide ahigher ratio of surface to via volume, countering this tendency. Theability to laser micromachine smaller via holes 102 in sinteredelectrolyte sheets 100 enables better quality filling and sintering ofthe conductive via fill material. Thus, it is preferable that via holes102 be less than 75 microns in diameter, and more preferably less than50 microns, with 0.3:1 to 1:5, and even more preferably 0.3:1 to 1:1 L/Daspect ratio. Most preferred are via holes with diameters of less than25 microns and aspect ratios of approximately ratio of 1:1.25. Thedrilling of small via holes with the preferred aspect ratios isadvantageously enabled by the above described laser micromachiningmethod according to the present invention.

With the small via hole diameters it may be advantageous to increase thenumber of via holes in order to maintain the resistance of the currentpath through the vias. Reducing the via hole diameter from 75 μm to 50μm entails a cross-section reduction of approximately 66%. Thecross-section reduction is defined as 1−(area of 50 um diametervia)/(area of 75 um diameter via). In this case, the number of vias maybe increased by a factor of 2 to 2.5 to compensate for the reduction inarea (cross-section). However, in many cases an increase in the numberof vias is not necessary because the via resistance is not a limitingfactor. In the case of vias with less than 50 μm diameters, it may bepreferred that clusters or groupings of smaller vias are used to replaceone single large via. For example, nine 25 μm vias holes have the samecross-sectional area as one 75 μm via hole and could effectively replacethat single via. These small via holes can be grouped in any number ofways including linear arrays of via holes, circular or rectangularcluster(s) of the via holes 102, or any pattern providing both goodmechanical integrity and adequate current distribution.

Multiple device manufacturing: Because mechanical cutting of sinteredelectrolyte would cause uncontrolled cracking and drastically reduce thestrength due to the defects created, mechanical cutting is typicallyperformed on green (i.e., unsintered) electrolytes. However, becausemechanical cutting requires the electrolyte sheets to be cut to size inthe un-fired state, only one fuel cell device can be fabricated persubstrate. The ability to laser cut or ablate the sintered electrolytesheet at arbitrary times during fuel cell device manufacture allows morethan one fuel cell device 150 to be fabricated on a single over-sizedelectrolyte sheet substrate 100. (See FIG. 3). After the fuel celldevices are complete, they can be cut out by using laser micromachining,and thus separated from one another. This approach can be used tofabricate multiple fuel cell devices in parallel to increase fabricationyield. If an over-sized electrolyte sheet is used, the fuel cell devicepatterns can also be shifted as needed to avoid electrolyte sheetdefects 101 and further increase yield. FIG. 3 illustrates schematicallythe concept of fabricating multiple fuel cell devices on a single,common, electrolyte sheet, and separating them (via laser cutting) afterthe last printing/firing step. The exemplary laser cutting path(ablation path) is illustrated schematically by an arrow 2 around one ofthe devices. The laser ablation path is creating a new edge surface asthe fuel cell device is being cut out. Thus, laser micromachining allowsthe fuel cell devices to be cut out of the electrolyte at arbitrarytimes during the fabrication process. Fabrication of multiple fuel celldevices on a single electrolyte sheet 100 reduces the number of handlingsteps by a factor equal to the number of fuel cell devices printedsimultaneously. Thus, although one fuel cell device may be printed at atime on a single sheet of electrolyte, it is preferred that two or morefuel cell devices 150 be printed on a single sintered electrolyte sheet100 and that the completed fuel cell devices 150 are laser cut away fromeach other after fabrication. That is, anodes and cathodes and otherlayers (if needed) are printed on a sintered electrolyte sheet 100, thevia holes are drilled and filled, and the electrolyte sheet 100supporting at least partially printed multiple fuel cell devices is thenfired (sintered). After sintering, the electrolyte sheet 100 is lasermicromachined, to cut out the completed or partially completed fuel celldevices 150. For small fuel cell devices, 10, 20 or more may be printedon a single electrolyte sheet 100 measuring 30 cm or more in width orlength. It is noted that the fuel cell devices 150 may be identical orof varying design. For example, the fuel cell devices 150 situated on asingle sintered electrolyte sheet 100 can be multi-cell devices, singlecell devices, or a combination of both. These fuel cell devices may beof the same size, or of different sizes and may have the same ordifferent aspect ratios (width/length).

Manufacturing multiple devices 150 on a single electrolyte sheet 100minimizes edge wrinkling, curling and unwanted thickness variation,because these problems occur mostly at the edges of the electrolytesheet, and not in-between the devices. In addition it minimizes thefrequency of device(s) handling and the amount of time each device ishandled, which leads to increased yield.

Minimization of electrolyte sheet edge curl and/or wrinkles: Lasermicromachining allows the electrolyte sheet to be over-sized in thegreen state, and than be cut to shape after sintering. If any edgecurling or edge wrinkling occurs while the electrolyte sheet sinters,the wrinkles and/or curl(s) can be cut off with the laser aftersintering. Material removal after sintering of the electrolyte sheeteliminates any need for electrolyte sheet stacking or for applyingweight to the electrolyte sheets during sintering, as a means ofreducing edge curl. It is preferred that the outer 1 mm to 5 mm regionof the electrolyte sheet be removed after sintering, in order to reduceor eliminate this edge curl. Trimming of over 1 mm off the edge of theelectrolyte sheet significantly reduces edge curl or edge wrinkling andsignificantly improves edge surface quality such as roughness along theedge face and probability of an edge crack, tear, or other feature thatcould concentrate stress. Although it is preferred that the minimumpossible amount of material is removed, more of the perimeter may beremoved, e.g., 3 cm or more, if needed.

Reduction in electrolyte sheet borders: Another benefit of using lasermicromachining to cut fully fabricated devices 150 from an over-sizedelectrolyte sheet substrate is a possible reduction of electrolyte sheetborder B around the screen printed electrodes 152 (see FIG. 4). Screenprinting of a particular electrode pattern (or electrode patterning ingeneral) requires a minimum electrolyte size with dimensions bigger thanthis pattern. The smaller the electrolyte sheet is, the more difficultit becomes to accurately align and print well defined features at ornear the electrolyte sheet edge. Using laser micro-machining, all of theelectrode layers can be printed/fired, and then the final device edgecan be cut arbitrarily close to (or through) the printed fuel cellelectrodes. For example, the resultant electrolyte sheet may have anunprinted boarder that is less than 5 mm, or less than 3 mm, or evenless than 2 mm wide. Reduction of electrolyte sheet borders allows newfuel cell device designs that were not previously possible, with theelectrodes existing up to the edge of the electrolyte sheet. Also, thiscreates less non-active area on the electrolyte sheet. The trimmeddevice 150 may even have an electrolyte sheet 100 that was lasermicromachined/trimmed to have no unprinted border(s).

Laser micromachining can be used to cut out or trim fuel cell devicesafter the final electrode, via fill, or busbar have been deposited andsintered. This allows fuel cell device(s) to be fabricated with muchless unprinted (non-active) electrolyte border along the edges than istypically practical for screen printing or handling. For example, lasercutting the perimeter of fuel cell devices after the final fabricationstep allows much greater surface utilization of the electrolyte sheet.Over the electrolyte surface, more area can be covered with functionalelectrodes and contacts if laser cutting/micromachining is used.Similarly, screen printed patterns of electrodes or other additionallayers can be made all the way to the edge of the electrolyte withoutconsideration for handling during processing. Typically, in fuel celldevices, only a small portion of the area near the edge may be printedwith electrodes or other components due to difficulties with fuel celldevice holding and handling. The method of the present invention enableselectrodes, bus bars, leads or other components of the devices to cover5% to 100% (FIG. 4) of the perimeter of the electrolyte sheet 100, to adistance B less than 5 mm of the edge of the electrolyte sheet, or evento the very edge. Laser cutting the edges of fuel cell devices enablesthe additional features to occupy much more of the device perimeter andallows them to exist much closer to the electrolyte edge than when othercutting methods are used. In the case of laser cutting, any additionalelectrolyte required for handling or processing concerns can be cut offafter the device fabrication is complete.

Laser cutting/micromachining enables the final fuel cell devicedimension to be created after mounting the fuel cell device 150 in or toa frame or manifold structure, or in an assembly of multiple fuel celldevices. For example, the electrolyte or fuel cell device can be lasermicromachined after it has already been mounted to a frame. This enablesa larger device to be carried through the process for handling purposes,including mounting, and then trimmed afterwards to remove the excess.Thus, according to some embodiments of the present invention the lasermicromachining method according to the present invention would be usedafter the mounting of a larger than needed fuel cell device on a frameor in an assembly of multiple devices and then laser cutting or trimmingthat larger fuel cell device after mounting it to the desired finaldimension.

Surface Pattern Machining: Laser micromachining can be utilized toperform surface pattern machining. Surface machining of electrolytesheets and/or fuel cell devices includes texturing, roughening, andmicro-windowing. Micro-windowing is the process of creating very thinregions in the electrolyte surface. For example, surface patterns 105,such as micro-windows 105′ can be laser micromachined into the sinteredelectrolyte sheet 102 opposite previously printed electrodes (anodes,cathodes) or other layers of the fuel cell devices 150. In this way,thinner electrolyte sheet windows can be created than is possible withmolding techniques, because, molding/casting techniques require aminimum electrolyte thickness in the un-fired (i.e., green) electrolytesheet to survive releasing of the green sheet from the carrier ontowhich it is cast. Cathodes or other layers can then be printed on thelaser micromachined areas, so that the windows will be sandwichedbetween the anode-cathode pairs or between other printed layers (suchas, for example, busbars, or catalyst layers). An embodiment of surfacemicromachining/patterning is described, for example, in the embodimentof Example 9 and is illustrated in FIGS. 26 a-26C and 27 a-27 d.

Laser cutting the final shape of the electrolyte after fabrication iscomplete allows quality control test structures to be fabricated inparallel on each electrolyte sheet 100. (For example, small test devicescan be fabricated on each electrolyte sheet next to the actual fuel celldevices 150. These small test devices will go through the samefabrication steps and conditions as the actual fuel cell device. Whenthe fabrication is complete, these small test devices (witness samples)can be cut off and evaluated. This allows device performance,non-destructive testing, and destructive testing to occur on these smalltest quality control samples instead of sacrificing an actual fuel celldevice. After fabrication, these structures can be cut off fordestructive or other testing. Mechanical cutting would require actualdevices to be sacrificed for testing or separate test devices to befabricated in series.

Examples Using ns Laser Configuration #1 Example 1a, 1b

In this laser micromachining system configuration (see FIG. 5),frequency-quadrupled ns Nd:YAG laser 160 from Lambda Physik Starline,GmbH. was used with an output wavelength of 266 nm, with an 1 kHzrepetition rate, and 2 mJ energy per pulse maximum to micromachine viaholes in a ceramic electrolyte sheet 100. A plurality of mirrors M_(i)direct the laser beam into an optical focusing lens L₁. The sinteredelectrolyte sheet 100 was supported by a movable XY stage S₁, and lensL₁ directed the focused laser beam onto the electrolyte sheet 100. Pulseduration of the laser 160 was 10 ns. Straight edges, via holes, andcurved patterns were micromachined in the sintered 20 μm thickelectrolyte sheet 100. The electrolyte sheet was substantially similarin composition and thickness as that described in US patent application2004/0265663. The depth of focus of the laser beam was about 300 μm. Itis noted that either a ps or fs laser system (providing similar or otherabsorbing wavelengths) can also be utilized. Also, lens systems creatingdepths of focus ranging from 1 μm to 1 mm can be used independent of thespecific laser system. These lens systems allow one to control the spotsize and thus the size of the micromachined features. Also, it allowsfor laser micromachining electrolyte sheets with surface heightvariations (e.g., corrugated or patterned surfaces). When lasermicromachining such electrolyte sheets, the height of the corrugationsor surface variations should be equal to or less than the depth offocus.

Example 1a

A plano-convex (PCX) lens L₁ with a focal length of 10 cm was used tofocus the light in proximity to the zirconia based electrolyte sheet. Asimple percussion drilling technique was used. The 266 nm laser 160 hadits optical power level set at 340 mW. This power level corresponds to340 μJ per pulse. Since the diameter of the via hole of this example isabout 50 μm, this gives a laser fluence level of roughly 17 J/cm². Inthe experiment the hole was laser cut/drilled through the electrolytesheet after less than 2000 pulses or 2 seconds. The minimum fluencelevel required to observe laser ablation effects (i.e. ablationthreshold level) was less than 6 J/cm², e.g., about 1 (0.9 to 1.1J/cm²). The via shape produced is influenced by the laser beam shape.Laser micromachining, without generating microcracks, was also achievedusing a range power levels of 100 to 600 μJ per pulse, and fluencelevels of 5 to 30 J/cm².

FIG. 6 a and FIG. 6 b are photographs taken with an optical microscopeof the exemplary micromachined via holes. Both the top surface of theelectrolyte sheet (i.e., the laser incident side in FIG. 6 a) as well asthe bottom surface of the electrolyte sheet (i.e., laser exiting surfacein FIG. 6 b) are shown. Re-deposition of ablated material from thegenerated plasma was observed on the electrolyte sheet in the form ofring patterns 108, but it is noted that re-deposition can be reduced,for example by utilizing ultrashort (<100 ps) pulsed lasers, by heatingthe electrolyte sheet 100 to elevated temperatures, or with the use ofpurge gas or debris collection chamber. FIG. 6 a and FIG. 6 b illustratethat although the re-deposited region is present (in the form of ringstructures around the laser micromachined via holes), there is noobservable micro-cracking. The mechanical integrity of the cut edges wasobserved in accelerated aging by exposing micromachined holes to watervapor at 105° C.-108° C. and 3.5-6.5 psi for over 115 hours. Noaccelerated transition to the monoclinic structure at the cut edgescompared to the bulk material was observed.

Example 1b

FIGS. 7 a and 7 b are SEM images of straight edges laser micromachinedwith the above described laser cutting equipment and at the samesettings, using a cut speed of 1 mm/s. Cutting speeds of 0.5 to 2 mm/scan be utilized, but the cutting speed was ultimately limited by thelaser repetition rate (i.e., max. speed is less than spot sizediameter×repetition rate). More specifically, FIG. 7 a shows a top viewof the laser micromachined edge surface and FIG. 7 b shows a side viewof the micromachined edge surface. Redeposition 108 is seen as adiscolored band near the micromachined edge on the laser incident sideis also apparent (see FIG. 7 a).

Examples Using ns Laser Configuration #2 Examples 2 to 4F

In another embodiment of a laser micromachining system for (nanosecond)laser cutting of sintered zirconia based electrolyte sheet, afrequency-quadrupled Nd:YVO₄ laser, made by Spectra-Physics(HIPPO-266QW), was utilized (Examples 2-3B). The output wavelength ofthis exemplary laser is 266 nm. The ns laser 160, running at arepetition rate of 30 to 120 kHz, has a peak laser power ofapproximately 2.5 W and a pulse duration of less than 15 ns according tothe specifications from the manufacturer. A 3× optical beam expander(BE) and with a 10.3 cm focal length telecentric lens L₁ were used inconjunction with the laser 160 to cut the electrolyte sheet 100. (FIG.8). Single pulse ablation on electrolyte test samples showed that thefocal spot size of the laser beam (beam waist) is about 20 μm indiameter.

Example 2

With an optical laser power of 1.7 W, light polarized parallel to thecutting direction, and a repetition rate of 30 kHz, a linear cuttingspeed of 40 mm/s was achieved with good reproducibility. The fluencelevel was calculated to be approximately 18 J/cm². SEM images of the(nanosecond 266 nm laser cut) electrolyte sheet edges of the electrolytesheet of this exemplary embodiment are shown in FIGS. 9 a-9 c. FIG. 9 aillustrates a cross sectional view of a laser cut edge. Note that thelaser ablated area on top and fracture surface at bottom. FIG. 9 billustrates the edge profile of a laser cut edge (receding away from thepicture). FIG. 9 c is similar to that of FIG. 9 a, but shows highermagnification of a laser cut edge cross-section. It also illustrateseffects of individual laser pulses in the upper ablation region.

The SEM images in FIGS. 9 a-9 c show that at the conditions used in thisexample, roughly 7 μm of zirconia material was removed by the laserablation process (ablation region 110 of FIGS. 9 a-9 c) before the restof zirconia electrolyte material was auto fractured by thermal stress(fractured region 112). The tensile thermal stress was generated by thetemperature difference between the top and bottom surfaces of thesintered ceramic electrolyte sheet. This is in strong contrast tofemtosecond laser cutting process, in which cutting is achieved byablating all the materials and negligible thermal effect is observed.Laser micromachining (Examples 2-4) produces an auto-cleaving orauto-fracturing effect that increases the cutting speed. Edge cuttingdemonstrations were performed with the ns laser with repetition rates of30 to 50 kHz and sample stage translation speeds of 25 to 40 mm/s. At 30kHz, according to some embodiments, the average laser power incident onthe electrolyte sheet was 1.7 W, and at 50 kHz the average laser powerincident on the electrolyte sheet was 1.5 W. Other methods of lasermicromachining defined features and creating stress to cut throughfracture the electrolyte sheets are also possible. For example, fs andother laser systems, system parameters, and applied external forces canalso be utilized.

FIGS. 9 a-9 c illustrate morphology characteristics of sintered andlaser cut edges. The laser ablated portion forms the edge bevel (region110) shown in FIG. 9 b. Nanosecond laser ablation of electrolyte isaccompanied by local melting (zone or region 114) with larger sizecrystals, as shown in FIG. 9 c. In FIG. 9 c individual pulse traces 116are clearly visible. The fractured portion of the material (region 112)exhibits grainy nature. The auto-fracturing process is caused by thermalstress generated by absorption of laser light in the material.

Example 3A

We have also examined the edge surface roughness obtained as a functionof cutting technique. Besides differences in the edge shape depending oncutting method as described in Example 2, differences in roughness ofthe edge face also exist. To observe these differences, edges cut undervarious conditions were evaluated using an optical interferometer. Theroughness of each edge face was measured over an area of 0.09 mm×0.01mm. These areas were selected to avoid the beveled corner (region 110)similar to that described in Example 2 and shown in FIGS. 9 a-9 c thatotherwise would have resulted in missing back-reflected data. Edge facescreated by laser micromachining sintered electrolyte with the fs laser(described in below as alternate laser configuration) and ns laser(described in Example 2) were evaluated. Also edges created bymechanically cutting and CO₂ laser cutting un-sintered electrolyte wereevaluated after they were sintered. All data points presented are valuesaveraged over 4 measurements from the same edge face.

FIGS. 10 a-10 c show the peak-valley, rms, and Ra roughness values forthe ns (266 nm) laser cut edge surface as a function of cutting speed.These figures illustrate that the faster cutting processes result inlower edge surface roughness. By adjusting the cutting speed, roughnessvalues of less than 5.5 μm (peak-valley), less than 0.4 μm (rms), andless than 0.3 μm (Ra) can be achieved. FIGS. 11 a-11 c show the edgesurface roughness values obtained on fs laser cut samples. FIGS. 11 a-11c display surface roughness data as a function of the laser power. FIG.11 a shows that the peak-valley roughness decreases as the laser powerdecreases. These values are typically higher than the ns laser edge faceroughness values due to the ablation caused by the fs laser. Theauto-cleaving or auto-fracturing process created by the ns laser cuttingcreates a smoother edge surface than that achieved by mechanicalcutting.

For comparison purposes, green electrolyte sheets were cut with (i) CO₂laser (10-6 μm) and (ii) mechanically with a knife edge, and thensintered. The electrolyte samples cut within CO₂ laser in theun-sintered state and then sintered had average edge surface roughnessvalues as low as: 13.04±1.21 μm (peak-valley), 2.52±0.17 μm (rms), and1.90±0.07 μm (Ra). Samples that were mechanically cut in the un-sinteredstate and then sintered had average edge surface roughness values as lowas: 5.63±0.79 um (peak-valley), 0.43±0.18 μm (rms), and 0.32±0.15 μm(Ra).

The cutting method described above yields an electrolyte sheet with anedge surface exhibiting greater than 10% ablation (see, for example,region 110 shown in FIGS. 9 a-9 c). Preferably the edge surface exhibitsbetween 50% and 90% fracture (FIGS. 9 a-9 c, region 112). The area offracture 112 is clearly differentiated from the area of ablation ormelting 110 (see FIG. 9 c) in that the fracture surface is straight,relatively flat, and perpendicular to the primary surface of theelectrolyte sheet as compared to the ablated or melted surface that ismore rounded and not perpendicular to the surface of the electrolytesheet. Less preferred is an edge showing partial melting of theelectrolyte surface. Also preferred is a device with an edge showingless than 20% of the circumference exhibiting fracture that deviates bymore than 100 microns (i.e. less than 20% is deviated by more than 100μm) from the ablated path of the laser. Deviation from this path is amis-cut and represents a flaw in the finished electrolyte sheet or fuelcell device. Improper laser power, repetition rate, or speed are primarycauses of this deviation.

Example 3B

This example demonstrates laser ablation related re-deposition.Referring to the optical microscope images of FIGS. 6 a and 6 bdescribed above, the re-deposition area 108 around the laser machinedvia hole is observed. The via hole was achieved by laser percussiondrilling with a 266 nm Nd:YAG laser with a 10 ns pulse duration and alaser fluence of 17 J/cm² described in ns laser configuration #1. Thecharacteristics of this re-deposition area can be controlled by varyingthe laser exposure conditions (wavelength, pulse duration, pulse energy,cutting speed, repetition rate), as well as sample temperature, andpurge gas and/or vacuum (e.g., their presence or absence, amount ofvacuum applied, amount and composition of purge gas), or otherparameters.

To gain more information on the re-deposition zone, an XPS (X-rayPhotoelectron Spectroscopy) analysis was performed. Edges that weremechanically cut in the green state and then sintered as well as edgescut with a ns laser after sintering were both evaluated. Specificallythe described ns laser configuration #2 (frequency-quadrupled Nd:YVO₄laser, Spectra-Physics HIPPO-266QW) was used to create the lasermachined edge at a cutting speed of 35 mm/s. FIGS. 12 a and 12 b showsthe XPS line profile data of the relative yttrium and zirconiumconcentrations at the sample surface as a function of the distance fromthe edge. FIG. 12 a shows that the zirconium level remains at a relativelevel of approximately 80% and the yttrium level is at a relative levelof approximately 20% within 1200 μm of the mechanically cut and sinterededge. FIG. 12 b shows XPS data from the same sample but from an edgethat was laser micromachined. This shows that the zirconium relativelevel is approximately 90% and the yttrium relative level isapproximately 10% within 200 μm of the laser machined edge. About 1000μm from the laser machined edge, though, these levels transition intothose observed at the mechanically cut edge. The re-deposited materialobserved near the laser micromachined edge has a higherzirconium-to-yttrium concentration ratio.

Examples 4A-4F

The laser micromachining system configuration for cutting and/ordrilling holes in sintered ceramic electrolyte sheets 100 used inExample 4A-4F is similar to that of Examples 2, 3A and 3B (shown in FIG.8). However, the laser micromachining system of Examples 4A-4F utilizesfrequency-tripled Nd:YVO₄ laser 160 with an output wavelength of 355 nm.Such a laser is available, for example, from Coherent, Inc. (e.g.,COHERENT AVIA-X). The laser micromachining system of Examples 4A-4F alsoincludes plurality of mirrors M_(i) that direct the laser beam to agalvo-scanner/f-θ lens. The galvo-scanner/f-θ lens is centered on axisZ, perpendicular to the XY stage S₁. (The galvo-scanner/f-θ lens isdenoted as lens L₁, which in this embodiment is Scanlab HurryScan 10scanner with a 100 mm focal length telecentric lens). During the lasermicromachining process, the sintered electrolyte sheet 100 was supportedby a movable XY stage S₁, and the lens L₁ directed the focused laserbeam onto the electrolyte sheet 100. The electrolyte sheet 100 wassubstantially similar in composition and thickness as that described inUS patent application 2004/0265663. The Nd:YVO₄ laser 160 has a M² value(M² is beam quality factor) of less than 1.3 and an output diameter of3.5 mm. In some experiments an optional 3× beam expander (BE) was usedto expand the laser beam provided by the laser 160. The nominal 1/e²beam diameter of the expanded beam was 10.5 mm. In these exemplaryembodiments the entrance aperture of the galvo scanner is 10 mm, so someclipping of the beam exists. The 1/e² focal spot size of the laser beamon the electrolyte sheet 100 was about 6.1 μm. Unless mentionedotherwise, the laser power and consequently laser pulse energy ismeasured on the electrolyte sheet surface.

More specifically, the thin, sintered zirconia based electrolyte sheets100 were laid flat on the XY stage S₁. The electrolyte sheets 100 wereproduced by a powder, slip, tape casting and sintering processes. Theseprocess produce electrolyte sheets 100 with one side appearing shinierthan the other which was the side. The shiny side of the electrolytesheet is the side of the sheet that touched the tape casting carrierfilm. Unless specified otherwise, laser cutting and drilling with themicromachining system of Examples 4A-4F were carried out with laserlight incident on the shiny side. Optimal focusing is achieved byadjusting the distance along the z-axis. Cutting is achieved bytranslating the electrolyte material with the XY stage. Drilling of viaholes was carried out using the scanner (i.e., by moving the focusedlaser beam relative to the electrolyte sheet).

Laser Cutting of Sintered Electrolyte Sheets Example 4A

In the laser micromachining system of Example 4a, a 3× optional beamexpander was utilized to expand the laser beam provided by the Nd:YVO₄laser 160. In this exemplary embodiment, laser beam pulse energy was 102μJ and laser pulse repetition rate was 50 kHz. The incident laser poweron the electrolyte sheets was 5.1 W. The laser beam was linearlypolarized with polarization vector at about 75° relative to the cuttingdirection. A cutting speed of 160 mm/s was achieved with cleanseparation of the electrolyte sheet pieces. The laser fluence level onthe sintered (ceramic) electrolyte material was about 350 J/cm². Thisfluence level is above the laser ablation threshold at the 355 nmwavelength. A total of 21 electrolyte sheet specimens with a dimensionof 2 cm×8 cm were prepared by cutting sintered electrolyte sheets withthe laser micromachining system of Example 4A and subsequently strengthtested using the 2-point bending method. The test results are describedlater on in the specification.

Example 4B

In the laser micromachining system of Example 4B, a 3× optional beamexpander was used to expand the laser beam provided by the Nd:YVO₄ laser160. In this exemplary embodiment, laser pulse energy was 95 μJ andpulse repetition rate was 50 kHz. The incident laser power on theelectrolyte sheets was 4.8 W. The laser beam was linearly polarized withpolarization vector at about 75° relative to cutting direction. Acutting speed of 120 mm/s was achieved with clean separation of theelectrolyte sheet pieces. The laser fluence level on the material wasroughly 330 J/cm². The fluence level is above the laser ablationthreshold at the 355 nm wavelength. A total of 29 electrolyte sheetspecimens with a dimension of 2 cm×8 cm were prepared by cuttingsintered electrolyte sheets with the laser micromachining system ofExample 4B and subsequently strength tested using the 2-point bendingmethod. The test results are described later on in the specification.

Example 4C

In the laser micromachining system of Example 4C, a 3× optional beamexpander was used to expand the laser beam provided by the Nd:YVO₄ laser160. In this exemplary embodiment, laser pulse energy was 21 μJ andpulse repetition rate was 125 kHz. Thus, the pulse energy in thisembodiment is about 5 times lower and the pulse repletion rate was about2.5 times higher than that of Examples 4A and 4B. The incident laserpower on the sintered electrolyte sheets was 2.6 W. Laser beam wascircularly polarized using a quarter-wave plate. A cutting speed of 100mm/s was achieved with clean separation of the electrolyte sheet pieces.The laser light fluence level on the sintered electrolyte sheet materialwas about 73 J/cm². The fluence level is above the laser ablationthreshold at the 355 nm wavelength. A total of 17 electrolyte sheetspecimens with a dimension of 2 cm×8 cm were prepared by cuttingsintered electrolyte sheets with the laser micromachining system ofExample 4C and subsequently strength tested using the 2-point bendingmethod. The test results are described later on in the specification.

Example 4D

The laser micromachining system of Example 4D utilized an unexpanded 355nm laser beam to cut electrolyte sheets (i.e., no beam expander isutilized). In this exemplary embodiment, the laser beam diameter at thefocusing lens was estimated to be about 4 mm. Laser cutting of sinteredelectrolyte sheets was performed with laser pulse energy of 194 μJ (onthe electrolyte material) and repetition rate of 50 kHz. Thus, the pulseenergy provided by the laser of Example 4D was higher than that providedby the lasers of Examples 4A-4C. The laser beam was circularly polarizedusing a quarter-wave plate. A cutting speed of 260 mm/s was achievedwith clean separation of the electrolyte pieces. The laser fluence levelon the material was estimated to be 108 J/cm². A total of 26 electrolytesheet specimens with a dimension of 2 cm×8 cm were prepared by cuttingsintered elect and subsequently strength tested using the 2-pointbending method. The test results are described later on in thespecification.

Separately 20 control 2 cm×8 cm electrolyte sheet specimens with weremechanically cut form the “green” sheet then sintered. They were alsotested using the 2-point bending method.

FIGS. 13 a-13 c illustrate an edge face of a laser cut surface of theelectrolyte sheet sample prepared using the laser micromachining systemof Example 4A. FIG. 13 a shows the cross-section of the micromachinededge face of the laser micromachined surface. FIG. 13 b shows thecross-section of the micromachined edge face surface at a highermagnification. FIG. 13 c illustrates the edge profile of a laser cutedge (receding away from the picture). The scribed depth (the depth ofthe laser cut groove) was about 8 μm. FIGS. 13 a-13 c illustrate thatsome deposition as well as melted material is present along the lasercut edge.

FIG. 14 a is an XPS profile showing the change in relative yttrium andzirconium concentrations as a function of distance from the laser cutedge produced by the micromachining laser system of Example 4a. The lineprofiles for laser-cut edge (FIG. 14 a) show that the relative Zr:Yratio changes from a value of about 92:8 at the edge to a value of about80:20 toward the center of the electrolyte sheet sample. In contrast,line profiles as a function of distance from mechanically cut edges(FIG. 14 b) of a control sample (cut in a green state, than sintered)show only a small amount of change in Zr:Y ratio which is about 80:20across the distance from the edge of the sintered sample.

FIGS. 15 a-15 c show SEM (scanning electron microscope) images of lasercut edges produced by the laser micromachining system of Example 4C. Inthis example, according to the SEM images, the laser scribe depth wasabout 13 μm or about 50% through the material thickness. Becauseincident laser power was only 2.6 W, the resulted tensile stressproduced by laser beam heating was relatively small. Hence theelectrolyte sheet material was scribed to a deeper depth than that ofExamples 4a and 4b, in order to enable the scribed electrolyte sheet toseparate or split by controlled fracture technique. In this example, theamount of laser ablation resulted in stress buildup, which in turnresulted in cracks running across the electrolyte edge face. Thesecracks can be detrimental to edge strength, and therefore are notdesired. FIG. 15 b also shows the columnar grain growth G (about 3 μm,vertically) that resulted from laser micromachining under theseconditions.

FIGS. 16 a-16 c illustrate the edge face of a laser cut electrolytesheet piece that was produced by the laser micromachining system ofExample 4D. The scribe depth (laser beam produced groove) was about 6 μmor about 23% of the total electrolyte sheet thickness. Columnar crystalgrowth of less than 0.5 μm in length was observed at the boundarybetween the melted layer and the unaffected material. The fractured edgeis very smooth and no crack formation was observed.

Applicants strength tested, using the 2-point bending method, all lasercut electrolyte sheet specimens produced by the laser micromachiningsystems of Examples 4A-4D. With respect to the laser incident surface,the cut specimens were tested with this laser incident surface both intension and under compression with different sample sets. The resultantedge strength data was plotted via a Weibull Distribution as shown inFIG. 17. More specifically, FIG. 17 shows plots of edge strength (MPa)vs. probability (percent) of failure. The laser micromachining systemconditions of Example 4D produced the highest strength values and lowestprobability (measured in %) of failure. The strength data labeled C1corresponds to the electrolyte sheet specimens produced by themicromachining systems of Example 4A; C2 corresponds to the electrolytesheet specimens produced by the micromachining systems of Example 4B; C3corresponds to the electrolyte sheet specimens produced by themicromachining systems of Example 4C; C4 corresponds to the electrolytesheet specimens produced by the micromachining systems of Example 4D,and the last two “sintered” data sets correspond to the measurements(shiny side under tension and shiny side under compression,respectively) of control samples (mechanically cut in the green state).When the laser cut test samples were put in tension, the lasermicromachining system of Example 4D (i.e., the system without the beamexpander) had the highest mean value of 1390 MPa. The lasermicromachining system of Example 4C (with the beam expander in place)yielded the lowest mean value of 805 MPa. For the edge strength resultsfor the laser cut test samples placed under compression, the electrolytesheet samples cut with the laser micromachining system of Example 4D,yielded the high mean value of 1698 MPa. The laser cut test samplesproduced by the laser micromachining system of Example 4C (Beam Expanderin place), under compression, yielded lower mean value of 790 MPa. Thelaser micromachining system of Example 4D is preferred since both thetensile and compressive strength of the laser cut edges produced by thissystem conditions is comparatively high.

The strength of laser cut specimens with the laser incident side undercompression (with the exception of those cut with the lasermicromachining system of Example 4C) was better than with the laserincident side under tensile stresses. This can be explained by theadverse effects of melting and heat-affected-zone around the laserablated groove which acts as fracture initiator. Placing the shinyelectrolyte side in compressive stresses for electrolyte specimens thatwere mechanically cut in a green state produced lower strength thanplacing the shiny side under tensile stress. This is because of edgecurl formed during the sintering process on the mechanically cutsamples. This edge curl is removed for the laser micromachined samplesduring the laser cutting after sintering, so the edge curl does notadversely affect the strength.

Laser Drilling of Via Holes on Sintered Electrolyte Sheets

Thermal effects need to be controlled and minimized during hole drillingof thin ceramic sheets (e.g., zirconia based electrolyte sheets) inorder to avoid microcracking. Preferably, this is done by minimizing theamount of power incident on the material, in order to decrease the levelof transient stress. The following examples show some of the exemplaryconditions which result in no microcracking around the perimeter of thehole.

Via holes can be drilled in sintered electrolyte sheets by using atrepanning technique. The thermal effects during the laser drillingprocess need to be carefully controlled (as described below for examplethrough pulse patterning) in order to avoid cracking of the material dueto thermal expansion. As such via hole drilling is typically performedover multiple passes around the desired profile. Multi-pass drillingtechniques can reduce the amount of thermal gradient buildup in thedrilling process, thus avoiding micro-crack formation during the process

In order to avoid cracking of the electrolyte sheets, the laser powerpreferably is reduced. Since the laser power is a product of laser pulseenergy and laser pulse repetition rate, this could be achieved byreducing either the pulse energy or the laser repetition rate. Inaddition, the scanning speed of the laser beam needs to be adjustedcorrespondingly based on thermo diffusion considerations.

Since observation of micro-crack at a single via requires high poweroptical microscopes or SEMs, an alternative technique of evaluating adrilling recipe is to drill a series of holes spaced closely togetherand observe whether crack forms between the holes. FIG. 18 a and FIG. 18b are two optical images of 4 holes of 60 um in diameter, spaced 250 umapart. Laser pulse energy of 180 μJ was used and approximately 25 passeswere needed to drill through the material. Initially at laser repetitionrate of higher than about 3 KHz and scanning speed of higher than 60mm/s, cracks along the holes were observed. FIG. 18 a is an opticalimage of the holes and the cracks running along the direction of holes.The conditions used were: a laser repetition rate of 4 KHz, a scanningspeed of 80 mm/s. As the laser repetition rate and scanning speed isreduced, cracks no longer form along the chain of via holes. This isshown in FIG. 18 b. The conditions were: a laser repetition rate of 3KHz, a scanning speed of 60 mm/s. Further analysis with SEM images ofindividual holes also showed no observable radial cracks along theperimeter of the holes

The above working example uses a low repetition rate to reduce thermaleffects. In FIG. 18 c is shown an example using low laser pulse energy.This shows an optical image of via holes drilled under conditions of 21uJ, a laser repetition rate of 15.04 kHz and a scanning speed of 600mm/s. A total of 40 passes were performed to drill through the material.

In order to obtain holes with smooth edges, it is important to considerhow pulses overlap between different passes. This is important when thevia hole diameter is considerably larger than the laser beam diameter atthe focus. In the following example, pulse patterns of a 60 um via holedrilled with multi-pass trepanning technique are analyzed.Experimentally it was found that with a speed of 60 mm/s and a laserrepetition rate of roughly 3 kHz, it takes roughly 25 passes to drillthrough the electrolyte material. The number of pulses per pass isroughly 10. In FIG. 19 shows pulse patterns as a function of number ofpulses per pass. The numbers of pulses per pass are: a) 10+0/25; b)10+1/25; c) 10+2/25; d) 10+3/25; e) 10+4/25; and f) 10+5/25. Clearlypatterns shown in FIG. 19 b through FIG. 19 e give smooth edges sincethe pulses were spread evenly around the perimeter, whereas FIG. 19 aand FIG. 19 f result in a hole with rougher edges. In general, knowingthe number of passes P, the best drill pattern with the smoothest edgeis obtained when the fractional pulse per pass is i/P, where Pi is notan integer, 0<i<P (i/P is a fraction that could be reduced or that i andP do not share a common factor). The number of pulses per pass could beoptimized either by change the scanning speed or laser repetition rateslightly.

Example 4E

The laser micromachining system used for drilling via holes in thisexample is the laser micromachining system of Examples 4A-4D. A 3× beamexpander was used to expand the laser beam. Laser drilling of via holesin a sintered solid oxide (zirconia based) electrolyte sheets wasperformed with a 355 nm frequency-tripled Nd:YVO₄ laser 160 providing 50μJ pulse energy and laser repetition rate of 10 kHz. Fluence level onthe ceramic electrolyte material was estimated to be 174 J/cm². Viaholes were drilled by scanning the laser beam with a galvo scanner.Multi-pass trepanning with a scanning speed of 100 mm/s approach wasused in this example. Approximately 10 to 20 passes were needed toablate through the sintered electrolyte material having a thickness ofabout 22 μm. FIGS. 20 a, 20 b are SEM photographs of laser drilled viaholes. FIG. 20 a is a SEM image of top-view of the via hole, whereasFIG. 20 b is a cross-sectional view. FIG. 20 b also shows a lip L formedfrom the melted ceramic (zirconia based electrolyte) material around thedrilled via hole. The lip height h is about 6 μm or 7 μm. No microcrackswere formed at the hole periphery. The lip L may be trimmed with a laserto a height less than 5 μm, preferably less than 3 μm, more preferablyless than 2 μm.

Via holes on the electrolyte sheets serve the purpose of allowing aconductor to connect cathode(s) to anode(s) through the electrolytesheet, thus conducting current between the electrodes through theelectrolyte sheet. Formation of lip around the periphery of the via holemay hinder current flow and act like a current constrictor, andotherwise create defects in subsequently formed layers of fuel celldevice(s). Thus, lip formation is undesired. It is noted that lipformation was not observed or lip height was insignificant with holesdrilled with nanosecond 266 nm lasers. The reason for the significantmelting and lip formation in this exemplary embodiment may be due to therelatively low photon energy of the 355 nm lasers. Unlike 266 nm photonswhich can potentially break the chemical bonds of the zirconia material,355 nm laser ablation is dominated by laser heating and melt evaporationmechanism. If the drilling process results in a significant lipformation, it is preferable to utilize a subsequent laser lip trimmingstep to minimize lip height.

Example 4F

The laser micromachining system used for drilling via holes in thisexample is the laser micromachining system of Example 4D. The lasermicromachining system of this example did not utilize the beam expander,accordingly an unexpanded 355 nm laser beam was used in this embodiment.The laser beam diameter on the focusing lens (L₁) was estimated to beabout 4 mm. Laser beam waist on the electrolyte sheet was approximately20 μm. Laser pulse energy was 194 μJ and the laser fluence on thematerial was about 108 J/cm². A hole with a given diameter of 60 μm wasdrilled (laser micro machined) through the sintered electrolyte sheetafter about 30 passes with a laser repetition rate of 4 kHz and atrepanning speed of 80 mm/s. Afterwards the lip was trimmed down with alaser trimming step was carried out by trepanning at a diameter of 90 umaround the same central location. The trimming step which involved 2passes at a speed of 80 mm/s with same laser parameters (same pulseenergy and pulse repetition rate). The main difference between thedrilling and trimming steps, were the laser path diameter(s). Thepurpose of the trimming step is to slightly ablate the lip formed duringtrepanning the through hole to a height h, where h is preferably lessthan 5 μm, more preferably less than 3 μm, even more preferably lessthan 2 μm and most preferably less than 1 μm.

A series of 5 via holes with a diameter of 60 μm were drilled with acenter-to-center spacing of 1000 μm. Again, these 60 μm diameter viaholes also incorporated an edge lip trimming step with a 90 μm diametercircle (laser beam waist of about 20 μm). Also, a series of 5 via holesof the same geometry were made with a center-to-center spacing of 200 μmwith the same trimming step. Finally, a series of 5 via holes with adiameter of 40 μm were formed that included an edge lip removal stepwith a 60 μm diameter. These were fabricated with a center-to-centerspacing of 200 μm. FIGS. 21 a, 21 b are SEM images of via holes drilledby laser trepanning, after the associated laser trimming. FIG. 21 a is atop-view of the via holes, whereas FIG. 21 b is a cross-sectional viewof the hole. Note the characteristic swirl pattern shown in FIG. 21 a,formed by the scanning laser beam. The swirl pattern was caused byrepeated heating and evaporation by the scanning laser beam of the meltpool formed by previous scans. From FIG. 21 b we can see that the lipformation was minimized. Via holes with reduced lip height h such asthat shown in FIG. 21 b are desired for use in solid oxide fuel cellapplications.

Although a laser trimming technique was used to reduce lip height in thedrilled electrolyte samples of Example 4F, other techniques such asspiral drilling and percussion drilling also showed promising results.In FIGS. 21 c and 21 d we present SEM images of percussion drilled viaholes with a diameter on the laser incident side of about 40 μm and adiameter on the laser exit side of about 10 μm. The lip height h wasabout 5 μm. Another method is the spiral drilling technique. FIGS. 21 eand 21 f show images of SEM holes drilled with spiral drillingtechnique. The lip height was about 9 μm, as shown in FIG. 21 f. Liptrimming, as discussed above, or further process improvement could leadto a decrease in lip height.

Examples Using ps Laser Configuration Example 5 Example 5

A picosecond laser was used to micromachine via holes on electrolytesheets. The laser had a 10 ps pulse width, a wavelength of 355 nm and apulse energy of 28 μJ maximum at a repetition rate of 100 kHz. The laserwas capable of a repetition rate range of 50 kHz to 2 MHz and a maximumpower of 4 W. A lens with a focal length of roughly 8 cm was used tofocus the light in proximity of the electrolyte sheet. The focal spotsize was estimated to be roughly 50 μm. Hence the laser fluence at thefocal point was roughly 1.4 J/cm². A percussion technique was used tomicromachine via holes in the electrolyte material. Microcracks wereobserved to be present at various laser repetition rate and powercombinations. Such cracks are not desired in the fuel cell device.Microcracking was not observed in the previously given UV lasermicromachining examples with fluences greater than 1.5 J/cm² and willnot be expected if the ps laser had provided higher fluence levels, forexample fluence levels similar to those provided by other examples (nsand fs configurations).

Examples Using fs Laser Configuration Examples 6 to 9

In this configuration, an amplified fs laser system (1 W Spectra PhysicsSpitfire® Pro Ultrafast Ti:Sapphire Amplifier) was used. The laseroutputs a 1 kHz pulse train at a maximum energy of 1 mJ per pulse. Pulseduration is approximately 40 fs, and the laser emission is centered at awavelength of 800 nm. A piano-convex lens with a focal length of 7.5 cmwas used to focus the laser light in proximity of the electrolyte sheet.Based on the laser Gaussian beam quality M² value of 1.4, wavelength,and a beam size of 7 mm (collimated beam diameter), beam waist at thefocal point was calculated to be 15 μm. Cutting trials were performedbelow the white light generation threshold which was found to be 35μJ/pulse with this lens system. Other focal length lens systems are alsopossible such as 3.5 cm or other options. Laser cutting speeds of 0.5 to2 mm/s were achieved without detrimental effects, but the cutting speedwas ultimately limited by the laser repetition rate. No microcracks atthe micromachined edge were observed.

Example 6

Micromachining can be used to reduce or eliminate effects of electrolytewrinkling including edge wrinkling that occurs during sintering. Onlarge electrolyte sheet pieces, for example of dimension greater than 10cm in width or length, wrinkling of the edges and other non-planarityhas been observed during the electrolyte sintering process. Theseeffects tend to be more pronounced as the electrolyte dimensionincreases. Depending on the electrolyte sheet size, these effects havebeen observed up to 4 cm within the electrolyte sheet edges. Lasermicromachining allows these large electrolyte sheet pieces to beover-sized during the sintering step. Laser micromachining can thenlater be used to cut off this excessive edge wrinkling, non-planarity,or any other defects that might exist after sintering or devicefabrication. To demonstrate the capability of improving the electrolyteflatness with laser micromachining, the fs laser was used to remove awidth of 2 mm from the electrolyte sheet edge. The edge flatness of asintered electrolyte sheet was measured before and after micromachining.FIG. 22 shows the surface contours as measured by a laser profilometerof approximately 20 μm thick electrolyte sheet before (top graph) andafter micromachining (bottom graph),—i.e., after laser cutting/removinga 2 mm wide perimeter from the electrolyte sheet edge. The electrolyteedge after sintering has a measured maximum height variation of 80 um,and the electrolyte edge has a much lower measured maximum heightvariation of 40 μm after laser micromachining.

Example 7

To demonstrate laser micromachining of solid oxide fuel cell devices(SOFC devices) 150, edges and vias were micromachined in a sinteredelectrolyte sheet 100 for both 10-cell and 1-cell devices. Both types offuel cell devices required multiple via rows to interconnect theelectrodes created on both sides of the electrolyte. Four differentdevice fabrication scenarios were demonstrated and are disclosed below:

1. Drilling holes in mechanically trimmed, bare sintered electrolytesheet. Bare electrolyte sheet was received that was mechanically cut andsintered (the electrolyte sheet had at least one dimension greater than10 cm dimension, which was required for a 10-cell device). Precisealignment of edges to the via holes, as they were drilled was required.Thus, while eleven rows of via holes were laser micromachined in theelectrolyte sheet using a laser trepanning technique (an example of suchhole is shown in FIGS. 23 a, 23 b), their placement was accuratelyreferenced to the as-formed mechanically cut electrolyte sheet edges.For trepanning, the laser beam was kept stationary and the sinteredelectrolyte sheet 100 was moved along a circular path. Via hole(circular) geometry was similar to that shown in FIGS. 23 a and 23 b.The cutting speed was limited by the low repetition rate of the lasersource. From FIG. 23 a and FIG. 23 b we can see that the quality of thevia hole 102 is very good. There remained some ring cracks 118 on theback side of the via hole 102 due to, presumably, shock waves generatedduring the laser ablation process. At higher average power of about 35mW, the ring cracks can be eliminated. With a cutting speed of 0.5 mm/s,the 60 μm diameter via holes 102 were laser cut in typically 2 passeswith an energy of 30 μJ/pulse and a fluence level of 17 J/cm². The10-cell device fabrication was then completed including steps of formingthe anode, cathode, current collector, via conductors, and busbarstructures.

The specific sample shown in FIGS. 23 a and 23 b was micromachined usingthe fs laser but with a pulse energy of 7 μJ/pulse and a fluence levelof 4 J/cm². In contrast, the actual 10-cell and 1-cell devices 150described previously had laser micromachined via holes using the samefs. laser but at 30 μJ/pulse and a fluence level of 17 J/cm². Thesehigher pulse energies and fluence levels reduced the formation of ringcracks and only 2 passes were needed to machine a through hole with thesame circular geometry. Although microcracking was observed when the fslaser fluence was 4 J/cm², the previous visible-near infrared lasermicromachining examples given with fluences larger than 4 J/cm² showedno observable microcracking.

2. Laser trimming and hole drilling in sintered oversized electrolytesheets. We received a bare sintered electrolyte sheet that was sizedlarger than required for a 10-cell device. In one embodiment theelectrolyte sheet dimensions were 12 cm×15 cm. Both the 11 rows of viaholes and the device perimeter were laser micromachined using a 800 nmlaser. Since the electrolyte sheet was oversized, no precise alignmentto the as-formed edges was required. Thus, only coarse alignment wasneeded. Approximately 1 cm to 1.5 cm was removed (micro machined) fromthe electrolyte sheet edges during the precision laser perimetercutting, and the via holes were accurately aligned to the perimetercut/micromachined edges. FIG. 24 a shows SEM images of edges that weremechanically cut in the green state and then sintered, and images ofedges that were sintered and then cut with the femtosecond laser (seeFIG. 24 b). The surface quality of the produced edge surfaces wassimilar to that shown in FIG. 24 b. FIG. 24 a shows, for comparison, across-section of the mechanically cut (while in the green state) andsintered electrolyte sheet. Measurements of the laser micromachinedelectrolyte overall length and width dimensions showed asample-to-sample variation limited by the measurement error of less than±0.04%. The 10-cell device fabrication was then completed includingsteps of forming the anode, cathode, current collector, via conduction,and busbar structures. In a prophetic example, electrolyte of 30 cm inlength is produced with via holes possessing a via-to-via registrationerror of less than 50 microns. This example demonstrates the edgeprofile and morphology characteristics of sintered and laser cut edges.Sintered laser machined edges such as what is shown in FIG. 24 b have aRMS roughness of about 0.4 to 0.8 μm. In case of femtosecond lasercutting of electrolytes, thermal effects were observed to be small suchthat the electrolyte can be cut through (ablated) without cracking.Re-crystallization of the vaporized and melted material result incrystal grain growth, similar to that shown in FIG. 24 b. The crystalgrain size is less than 1 μm (FIG. 24 b). At the same cutting speed,increasing laser fluence will result in crystal grain growth in size.

3. Laser hole drilling for multiple devices in a sintered oversizedelectrolyte sheet substrate, with subsequent cutting and separation ofthe drilled electrolyte sheets corresponding to these devices. A largebare electrolyte sheet dimension >10 cm was mechanically cut andsintered. Multiple clusters of 2 rows of via holes, each correspondingto a 1-cell device were laser drilled/micromachined in the largeelectrolyte sheet and multiple sections of the electrolyte sheet, eachcorresponding to a different 1-cell device with a dimension ≦5 cm werelaser micromachined out of the large electrolyte sheet. Thus, both the 2rows of via holes and the electrolyte sheet perimeters corresponding toeach fuel device were laser micromachined, and the vias andmicromachined edges were accurately aligned to each other. The 1-celldevice fabrication was then completed on each separate electrolytesheet, including steps of forming the anode, cathode, current collector,via conduction, and busbar structures.

4. Multiple device manufacture on a single oversized electrolyte sheet Alarge electrolyte sheet was mechanically cut and then sintered. Thiselectrolyte sheet had at least one dimension greater than 10 cm.Multiple anode patterns for a plurality 1-cell devices were previouslyprinted and sintered on one surface of the received electrolyte sheet.Laser micromachining of the sintered electrolyte sheet was utilized todrill multiple sets of two rows of via holes 102 (each set correspondingto a different fuel cell device) and the perimeters of the 1-celldevices (with dimensions ≦5 cm). The laser micromachined features wereaccurately aligned to the previously fabricated anode layers. Theresulting laser cut electrolyte sheets (corresponding to 1-cell devices)incorporated both previously fabricated anode pattern and an aligned viapattern. The 1-cell device fabrication was then completed includingsteps of forming the cathode, current collector, via conduction, andbusbar structures.

The following is an exemplary process for manufacturing multiple soldoxide fuel cell device on a single oversize zirconia based electrolytesheet:

-   -   a. Sintering a green electrolyte sheet (T≈1450° C.);    -   b. Printing anodes and other layers as needed and sinter        (T≈1350° C.);    -   c. Laser drilling via holes;    -   d. Filling via holes with conductive via material and sintering        (T≈1250° C.);    -   e. Printing other layers (e.g., cathodes), and sintering        (T≈1200° C.);    -   f. Printing bus bars, etc and sintering (T≈750-1000° C.);    -   g. Cutting out each (at least partially completed) fuel cell        device by laser micromachining after the last sintering step.

It is noted that the manufacturing starts with higher sinteringtemperatures and proceeds to progressively lower sintering temperatures.

Example 8

Edge strength of zirconia electrolyte is of great importance in someapplications. To demonstrate the strength obtained from the lasermicromachined edges, 2-point bend tests between parallel plates wereperformed with electrolyte samples approximately 2 cm×8 cm. The strengthof mechanically cut and sintered samples was measured as a reference.The mechanically cut samples were measured with the smoother surfacethat was cast against the Teflon carrier experiencing tensile stress.The samples with the micromachined edges were tested with the laserincident side in both tensile and compressive stress configurations.FIG. 25 illustrates strength of laser cut (micro-machined) edges,compared to mechanically cut edges. More specifically, FIG. 25 shows theWeibull distribution probability plots of the measured bend strength.Under one set of conditions, both the tensile and compressiveconfigurations of the laser incident side show similar strengthdistributions to the mechanically cut and sintered electrolyte. However,a second set of samples that experienced an increased vacuum force whilemicromachining exhibited a much higher strength when the laser incidentside was in compression. These higher strength samples had a vacuumchannel holding the electrolyte in position while cutting. The vacuumchannel (see FIG. 8) was aligned on the electrolyte side opposite of theincident laser power, and the vacuum force was pulling the electrolyteaway from the incident laser energy. One unexpected result is that theelectrolyte sheet cut with the fs laser under these conditions can showhigher strength than seen in any of the other mechanically cut or lasercut samples, greater than 2 GPa, 2.7 GPa, and even as high as 3 GPa,versus typical strength of approximately 1.0 to 1.5 GPa. The higheststrength parts were cut with while a vacuum was applied on theelectrolyte sheet 100 via vacuum channel(s) 165 during the cuttingprocess, pulling the electrolyte sheet 100 down, away from the laserduring cutting.

A general observation of the femtosecond laser micromachined samples isthat debris around the micromachined area was significantly less, incomparison with the nano-second laser micromachined samples. Thefemtosecond laser machined parts exhibit a unique surface morphologythat shows essentially complete ablation without substantial fractureand limited grain growth (e.g., grain size in 3YZ electrolyte was lessthan 2 μm, ad typically less than 1 μm). The unusually high strength ofthe electrolyte sheet edges is thought to be associated with this uniquemorphology. Electrolyte sheet with a laser micromachined edge surfaceexhibiting 100% ablation, or/and grain size of more than 0.2 microns butof less than 2 microns is preferred for strength optimization.

Examples 9a and 9b Surface Patterning Example 9a

This Example demonstrates the use of laser ablation for surfacepatterning of electrolyte sheets. Another identified application oflaser micromachining the electrolyte is for manipulating the zirconiasurface to produce roughened, textured, or micro-windowed patterns.Laser micromachining partially through the electrolyte sheet 100 allowssurface machining that may not be possible through molding or castingtechniques. For example, molding or casting techniques require a minimumelectrolyte thickness in the un-fired electrolyte to survive releasingit from the Teflon carrier. In some applications, it is preferred thatthe sintered bare electrolyte have a minimum thickness to survivehandling as a free standing film. For example, an electrolyte with athickness of about 20 μm can be laser micromachined after electrodelayers have been fabricated on it. FIGS. 26 a-26 c illustrate a 20 μmthick electrolyte substrate with a fired 5 μm thick anode layerfabricated on one side. As shown, laser micromachining is used topartially remove the electrolyte layer producing a window thicknesst_(w) (electrolyte sheet patterns 105) of less than 5 μm, which can notbe generally produced in a free standing or self supporting electrolytesheet. In this case, the existing anode layer provides the requiredmechanical strength to survive handling. A cathode layer is thenfabricated on the opposite side to complete the fuel cell device. Themicromachined features constitute a significant percentage of the areaunder the electrodes, preferably greater than about 25% and morepreferably greater than about 40%. The patterns may have a relief(depth) of greater than 5 μm or preferably more than 30% and morepreferably more than 50% of the electrolyte sheet thickness. The methodof the present invention is especially applicable for use withelectrolyte sheets with overall thicknesses <100 μm and preferably <30μm, most preferably less than about 20 μm, although it can be utilizedwith much thinner electrolyte sheets, even as thin as 3 or 5 μm. Thedescribed method is also applicable to laser cutting, laser drilling,and surface machining of the electrolyte sheet after additional layershave been applied.

Example 9b

This Example also demonstrates the use of laser ablation for surfacepatterning of electrolyte sheets. Laser micromachining the electrolytesurface with the fs laser system was used to create a 10×10 arraypattern of 50 μm wide squares. Spacing between the squares was alsoapproximately 50 μm. Each square was created by mastering the laserenergy for a total of 10 line scans offset by 5 μm each. The focallength of the lens was 35 mm and the laser power was 4 mW. FIGS. 27 a-27d show both optical microscope as well as optical interferometric dataof the micromachined features. FIG. 27 a shows a section of the 10×10array pattern as seen with an optical microscope. The slight roundingand enlargement of two of the corners of each square is apparent. Thisis due to the laser persistence at these points during the start andstop of each raster cycle. The average depth of the squares is 4.0um±0.1 um, and the optical interferometer depth image is shown in FIG.27 b. FIGS. 27 c and 27 d show optical interferometer images at thebottom of each square feature over a 0.04 mm×0.04 mm area. As shown, thepath of the laser mastering is observable. The average roughness valuesfor these laser micromachined surfaces was 4.88 um±1.22 μm(peak-valley), 0.35 um±0.04 μm (rms), and 0.26 um±0.02 um (Ra). Forcomparison, the values for the un-machined electrolyte surface are 1.231um±0.377 μm (peak-valley), 0.046 um±0.001 μm (rms), 0.034 um±0.001 μm(Ra).

The inventive method is applicable to fuel cell devices and specificallyto the electrolyte supported multi-cell design and fabrication process.The method is especially applicable to manufacture of fuel cell devicesthat are based on multiple cells fabricated on a common electrolytesubstrate and interconnected through conductive vias. Because, accordingto one aspect of the present invention, laser micromachining (e.g., holepunching and electrolyte trimming) is done after sintering, lasermicromachining is particularly useful for processing of large deviceswith a dimension over 30 cm where processing of ceramic in the unfiredstate would require highly demanding shrinkage control. One advantage ofthe method of the present invention is the increase in devicefabrication yield, throughput, and performance.

It will be apparent to those skilled in the art that variations andmodifications can be made to the present invention without departingfrom the scope of the invention. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A sintered electrolyte sheet comprising: a body of no more than 45 μmthick and at least one laser machined feature with at least one edgesurface having at least 10% ablation.
 2. The electrolyte sheet of claim1, wherein said at least one edge surface has more than 50% fracture andless than 50% ablation.
 3. The electrolyte sheet according to claim 1,wherein said electrolyte sheet has multiple electrodes situated on saidelectrolyte sheet.
 4. The electrolyte sheet of claim 1, wherein saidelectrolyte sheet is zirconia based and includes a micromachined edge,and the relative concentration of zirconium at the micromachined edge ishigher than at another area that is located at the surface of theelectrolyte sheet further from the laser micromachined edge.
 5. Theelectrolyte sheet of claim 1, wherein said electrolyte sheet has a lasermicromachined surface and the laser micromachined surface has averagecrystal grain size of less than 1 micron.
 6. The fuel cell deviceincluding electrolyte sheet of claim 1, and at least one anode andcathode pair, wherein said features are via holes with diameters lessthan 75 microns.
 7. The fuel cell device of claim 6, with length oflarger than 30 cm, and with part-to-part via hole registrationrepeatability of less than +/−200 microns.
 8. An electrolyte of claim 1having edge strength of greater than 1.8 GPa.
 9. The electrolyte sheetaccording to claim 1 wherein said electrolyte sheet is zirconia basedelectrolyte sheet with at least one edge surface exhibiting 100%ablation and grain growth at the ablated edge surface, wherein the grainsize of the ablated edge surface is less than 2 μm.
 10. The electrolytesheet according to claim 1 wherein said electrolyte sheet has anun-printed border, said boarder being less than 5 mm wide.
 11. A methodof micromachining an electrolyte sheet comprising: (i) supporting asintered electrolyte sheet; (ii) micromachining said sheet with a laser,wherein said laser has a wavelength of less than 2 μm, fluence of lessthan 200 Joules/cm², and repetition rate (RR) of between 30 Hz and 1MHz, pulse duration less than 1 μs.
 12. The method according to claim 1,wherein said laser is a 355 nm nanosecond laser.
 13. The method ofmicromachining an electrolyte sheet according to claim 23, wherein saidlaser has a pulse duration <1 μs, wavelength <400 nm, and: fluence isbetween 5 Joules/cm² and 200 Joules/cm², repetition rate (RR) of atleast 1 KHz, and cutting speed >50 mm/sec.
 14. A method of makingmultiple fuel cell devices, wherein multiple fuel cell devices (i) areat least partially fabricated on a single sheet of electrolyte; and (ii)are laser cut, separating them from one another according to themicromachining method of claim
 11. 15. The method of micromachining anelectrolyte sheet according to claim 11, said method including cuttingoff by micromachining more than 1 mm of at least one side of theelectrolyte sheet, so as to remove at least a portion of electrolytesheet edge curl.
 16. The electrolyte sheet of claim 11, wherein themicromachining is performed by ablation in conjunction withauto-cleaving.
 17. The method of micromachining an electrolyte sheetaccording to claim 11, wherein said micromachining includes cuttingholes or trimming perimeter of the electrolyte sheet trough its entirethickness.
 18. The method of micromachining an electrolyte sheetaccording to claim 11, wherein said electrolyte sheet is a corrugatedsheet and said micromachining method produces via holes in saidelectrolyte sheet.
 19. The method of micromaching an electrolyte sheetaccording to claim 11, including a step of laser micromachining at leastone additional layer situated on said electrolyte sheet.
 20. The methodaccording to claim 17, wherein said holes are produced with a lip andsaid lip is trimmed off with a laser to a height of less than 5 μm.